How bad is this for Trump, and can the GOP weather the storm? Bet on yes because of the stability of the two-party system, but, uh... uh.... uh...
Let's think of this in the context of Watergate. The "Watergate" of all scandals ending in "-gate."
Let's start, though, with what we know about what's happening today. Russia hacked the DNC's emails in an attempt to help Trump get elected, while Trump was suggesting that he wouldn't necessarily defend our NATO allies if Russia attacked them. He also urged Russia to engage in more espionage on his behalf. After winning, the outgoing President imposed light sanctions on Russia for their actions. Trump's campaign flunky and soon-to-be-National Security Advisor contacted a Russian envoy, on a monitored line (because Flynn, like Trump, is a fucking moron) to tell them not to respond because Trump would go easier on them, thereby engaging in private diplomacy in violation of the Logan Act, undercutting US foreign policy. Putin responded by... doing nothing. Trump praised Putin, like he always does.
The DoJ informed Trump that Flynn pulled this shit, and could be open to Russian blackmail. This contradicts what Pence had said when he publicly went to bat for Flynn. Trump did what with this information? Nothing. Until the other day's WaPo story came out, forcing his hand. And all of this was happening while the Trump campaign was in constant contact with Russia before the election took place.
This is what we know now. Suppose everything in the dossier is bullshit. Or some other fluid. But add the fact that the dossier exists.
Watergate cost the Republicans a bunch of seats in Congress in 1974. They then lost in 1976. Strategically, Republicans are in a bind right now. The impulse to circle the wagons around the President to prevent Watergate-or-worse losses is understandable. If every Republican declares the attacks on the Trump administration to be merely partisan, then it becomes difficult for anyone but the most highly informed voters to tell what is really true, and informed voters are generally partisans anyway.
The danger, though, is that we still don't know exactly where this goes. Social science is about analysis of patterns. We are so far outside of historical precedent here that social science is of minimal value right now. This doesn't even read like Watergate. It reads like a bad spy novel. A really bad one. The kind with characters who are too stupid to live.
Bad writers are usually predictable. Not this time.